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Unintended consequences of poor regulatory conditions.
For the most part, the only ones that "know" what their vehicles are getting for fuel economy are relying on what the on-board computer is telling them. In my experience, those on-board computers typically lie by about 10% optimistic. I've had 3 BMW's, a MINI, and two VW's with on-board computers. All but one read about 10% high, and that one was one of the BMWs (2005 330ci).Using actually tank-to-tank records for MPG, the only vehicles that I've been able to get EPA MPG numbers on a regular basis have been the two VW TDI's that we have now. The 3 normally aspirated BMWs and supercharged MINI all had to be babied to get EPA numbers.But, if I relied on the computer, then, I'd thinking I was meeting or exceeding EPA numbers.
Case in point, Ford's Powerstroke turbo diesel engines. The 7.3L power stroke, circa 1997-2003 was capable of returning 18-19 MPG in a 7,500 lb F250.Then along comes tougher EPA regulations on emissions of NOx (oxides of nitrogen, NO, NO2, etc). The easy solution was EGR, which reduces these emissions by lowering combustion chamber temps. Of course, lower combustion chamber temps also cuts efficiency. The resulting 6.0L power stroke (2003-2007) made more power than the 7.3L (by using higher boost), but, returned about 15% lower fuel economy, and had additional complexity in the form of the EGR system, and its accompanying EGR cooler.The lower combustion temps, in addition to lowering efficiency, also caused production of my particulate emissions (soot). Well, gotta fix that now. So, for the 2008 model year, the EPA required diesels to be fitted with a DPF (Diesel Particulate Filter), which requires a "regen" cycle to clean it, which essentially burns extra fuel to get the DPF real hot, thus burning the trapped particles to a (supposedly) less harmful gas.The result is the 6.4L Powerstroke of 2008-2010 gets about 15% less fuel economy than the 6.0L of 2003-2007, bringing it down to the same as the V10 petrol powered engine, but, much more expensive and complicated.For the 2011 model year, NOx emissions standards got tighter again. So, now, most diesel vehicles (including large commercial trucks) are fitted with DEF injection (Diesel Emissions Fluid, urea). This did allow the amount of EGR to be reduced, in turn reducing DPF regen cycles. The results of those changes brought the fuel economy of the 2011 6.7L power stroke back to that of the 1998 7.3L. How's that for progress.Also, consider if you will the false savings in emissions of these various targeted standards that, not only cause more fuel to be burned, but, also INCREASE emissions of green house gasses CO2 and CO ("carbon emissions"). The EPA measures these in "parts per million", which is a percentage of total exhaust gasses, so, in the EPA's eyes these numbers didn't get worse, but, any way you measure it, more fuel burned = more carbon emissions, and also more costs, not only in terms of fuel cost for the end user, but, the total environmental cost of extraction, refining, and distrobution of said fuel.Another thing to consider is the considerable complexity these emissions systems have added to vehicles, which translates directly into higher costs, both at time of purchase, but, also maintenance and repair. The same can be said of turbo petrol vehicles. Another example of this is the VW TDI. The 2006 Golf TDI was good for 50 MPG highway. It didn't have a DPF. The 2009-2014 Golf TDI has a DPF, and is only rated for 42 MPG highway.
I'm gonna throw a theory out that the computers are likely more accurate than your calculations.Why?1. Talk to someone from the bureau of weights and measures (we've had a member chime in before) has pumps may easily be slightly out of calibration.2. What's the chance that the auto fill stops at the same point every time?VsAn ECM so precise in measuring fuel delivery that it holds the air/fuel ratio in a very narrow band.I'm thinking YOU are more fallible.My experience with personal owned 2 Jeeps, 2 Subarus, 1 VW, 1 Mini, 1 Nissan (not to mention years of press fleet vehicles) is the mileage read outs were always close, often < +/- 1 mpg.I find it hard to believe there's a pattern of straight optimistic readings. That smells of personal tin foil beanie bias bra.:-*
The Breva I ride always shows my milage about 2-3 mpg lower than what I get when I do it the old fashion way.
I'm gonna throw a theory out that the computers are likely more accurate than your calculations.
I'm using longer-term calculations over several tank fulls to help eliminate the errors of getting the same top off every time.Any error in attaining a precise top-off would cancel out over many tankfuls.Yes, the ECM *can* calculate fuel used exactly, the same as speedometers *can* calculate exact speed, but, are often 3-5% optimistic. Add to that countless forum posts to the same (Ford Powerstroke forum being one of them).
Funny, the MPG readout on my Breva 1100 is usually about 2-3 MPG HIGHER than wha tI get calculating it the old fashion way (again, like with my cars, tracking and averaging many tankfuls).
Serious question . Is EPA mileage testing done with straight gasoline as opposed to ethanol blends ? Dusty
Maybe in some or many cases, but not in my own experience. We keep log books of all the gasoline that goes in each car and the miles they go. Our 2004 Taurus' average MPG readout consistently shows 20% lower than my log book calculations, which when averaged over the same tankfuls, pretty well removes the problem of exactly where the pump shuts off.In case anyone wonders why we bother keeping logs, I find it helps me know if a car needs attention, particularly if it's one that I don't drive very often.
I BELIEVE it is...
Well, that could account for some of the variation in EPA and real world mileage figures . Dusty
True, though it wouldn't account for CR's comparative variances, nor our own NA model that has been beating epa ratings o n E10 for about 3 years/36k miles.
Nah, several tankfulls just allows you to potentially increase the inaccuracy no? Especially if you're a creature of habit and use the same station/pump multiple times.
And forums are probably filled with tons of guys making bad calculations or with agendas or...PS, I equally doubt all assumption that speedos are optimistic. Many late-model vehicles have GPS tied in and you can tell right away. Plus optimistic speedos would normally be coupled with optimistic Odo's which would open the manufacturer to warranty law suits.
WRONG. You're forgetting which one of us is an engineer. I log, track, and analyze data as part of my JOB.By doing this on EVERY tankful, I get total gallons over total miles.For example, if top off, then drive 10,000 miles on, and top off again, and I put 250 gallons in over those 10,000 miles, my average MPG for the 10,000 miles is 10,000 / 250 = 40 MPG. It does matter if I topped off every tank during that time at all, so long as I kept track of EVERY fill during that time.Actually, in many cases the speedo is optimistic, but the odo accurate. This is, supposedly be design, so that if there is a real speedo error, someone doesn't end up getting a ticket and suing. If the odo is accurate and the speedo optimistic, it HAS to be designed that way, at least with electronic speedos. In the days of mechanical speedos, the discrepancy could've been do to wear.
Kev , I'm guessing that if Jay has logged and averaged mileage for 10,000 miles the figure he is coming up with is damned accurate . Unless he only buys fuel at the very same pump every time , the other variations will average out over that data period . Of course , I enjoy a good squabble twixt brothers , so carry on ;D Dusty
Perhaps, but it makes a lot of assumptions, odo accuracy alone can swing it one way or the other no?Why is my theory the least plausible when all it really is boils down to the thought that WE as humans are the least accurate. That it's just as likely the factors we are using to log and calculate mpg are at least as flawed as the ECM based calculations from which we are seeing deviations!
So... if Jay starts charting all of his trips , and measures his odometer readings against GPS mileage figures , and only makes right turns on the second Tuesday of each month while driving with his windows half way up , then will you believe his numbers ? Huh , huh ;D Dusty
Well, it seems that YOU as an engineer seem to doubt the work of OTHER engineers who came up with the mpg monitoring programs so I'm in good company. :-*The problem with your objection is your calculations ASSUME the accuracy of inputs not proven to be so.For instance you're ASSUMING the number on the pump is never wrong.Why is it that only the speedo and mpg calculators are suspect but not the pump and odo?
EPA's test fuel for light-duty gasoline-fueled vehicles does not currently contain ethanol or other oxygenates. However, EPA does account for the impact of low-level ethanol blends in our fuel economy estimates. Ethanol has a lower energy density than gasoline—about 1/3 less energy per gallon. That means a car operating on E10 (10 percent ethanol and 90 percent gasoline) would require about 3 percent more fuel to travel one mile than a car operating on gasoline and thus have about 3 percent lower fuel economy. EPA adjusts the fuel economy label estimates downward by about 10 percent to account for a variety of factors that are not currently accounted for during laboratory fuel economy testing, such as tire under-inflation, wind, hills, and road conditions. It also includes a 1.5 percent downward adjustment to account for the average national ethanol content.
I don't so much doubt the work of OTHER engineers, but, suspect the motives of the corporations themselves ($$$).
.... Until his last two vehicles he tended to buy used with at least 50k on the odo, while I tend to sell by that point. How does that effect the fuel system or drive line with regards to accuracy of not only fuel metered, but miles traveled?
Which brings us back to my tin foil beanie comment.... ;)
Which brings us back to my tin foil beanie comment.... ;)In related news I finally checked on the new Eco Boost Ford motors and it does look like they can at least run on regular and not premium. That's a plus.